The present invention pertains to a fuel-operated air heater, especially an air heater operated independently from the engine for motor vehicles, according to the preamble of patent claim 1.
Air heaters are monitored by sensors to guarantee reliable operation. Distinction is made between two functions:
flame monitoring
overheating monitoring
The flame monitoring serves the purpose of detecting a successful start and of reliably and rapidly recognizing flame blow-off.
The overheating monitoring is used to rapidly detect an unacceptably high component and heating air temperature, which is limited by law, as a consequence of the damming up of heating air or other disturbances.
It has been known that these functions can be monitored with two separate sensors. The flame sensor typically detects the temperature of the heat exchanger at a point suitable for this.
Prior-art overheating sensors detect a mixed temperature consisting of the component temperature and the heating air temperature at another point of the heat exchanger. Compliance with the heating air temperature limited by law can thus be ensured only with a lead with great temperature differences.
Another drawback of such an overheating sensor is that due to its design and the connection to the mass of the heat exchanger, it responds to suddenly occurring overheating conditions relatively sluggishly. Moreover, this effect is amplified by the design of the sensor. The sensor element is often seated in an enveloping tube and is separated from same by an air jacket. The signal is, moreover, also affected by the heat radiation of the heat exchanger and it therefore also depends on the heating output.
The object of the present invention is to create an air heater with overheating sensor and flame sensor of the type described in the introduction, in which the overheating sensor has a very simple design and is extensively insensitive to variations in the diameter of the outlet scoop at equal heating air mass and is arranged in the heat exchanger such that rapid detection of the particular measured quantity is reliably guaranteed.
The basic object of the present invention is accomplished by an air heater of the type described in claim 1.
The air heater is advantageously improved by the features of the dependent claims 2 through 15.
The essence of the present invention is that the overheating sensor is arranged as a heating air temperature sensor in the area of the heat exchanger near the blower. In the case of a combination of an overheating sensor and a flame sensor, the function is also guaranteed, especially in the case of short heat exchangers, in an area near the dome.
In particular, the overheating sensor is arranged in the heating air flow without being in contact with the wall.
An especially simple design and rapid response to temperature changes are obtained if the overheating sensor is designed as a nonencapsulated heating air temperature sensor.
The overheating sensor is preferably designed as small plates or as cylinders, which extend in the axial direction of the heat exchanger or at right angles thereto. In the case of a small plate extending transversely, the thickness of the plate also defines the leading edge.
The overheating sensor may be fastened, especially clipped, in a bracket provided, especially cast integrally, on the heat exchanger jacket.
The overheating sensor, the overheating switch and/or the flame sensor are preferably accommodated in a common holding means.
The signal of the overheating sensor can also be advantageously used to evaluate the degree of damming.
The overheating sensor may also be used in conjunction with at least one component sensor.
The signal of the flame failure controller is advantageously used as a redundancy in addition to the overheating sensor.
The case of overheating is detected with a heating output-dependent temperature characteristic.
The greatest temperature gradient is indicated by the overheating sensor according to the present invention.
Compliance with legal regulations concerning the limitation of the surface temperature and the heating air temperature is reliably and effectively guaranteed by the present invention.
Two cases are to be distinguished concerning the recognition of damming:
1. Creeping damming/partial damming
Creeping damming is characterized, e.g., by a slow clogging of an intake-side wire grid and is accompanied by a drop in the heating air mass. Partial coverage of the heating air inlet takes place in the case of an abrupt partial damming.
2. Abrupt complete damming
Complete coverage of the intake-side wire grid with impermeable material takes place in the case of the abrupt complete damming.
Both cases can be detected according to the present invention with a heating air temperature sensor at the blower-side end of the heat exchanger:
1. There is an essentially heating output-dependent temperature difference from the limited heating air discharge temperature (150xc2x0 C.). Thus, such a case of overheating can be detected with a heating output-dependent characteristic. The dome temperature remains markedly below the maximum of 350xc2x0 C. specified for the change in temperature in all cases of damming if the design of the heat exchanger is favorable.
2. The temperature measurement point shows the greatest gradient, as a result of which the most rapid recognition possible of complete damming is possible through the evaluation of the gradient.
When the damming is eliminated, the temperature also drops again most rapidly there. Since the damming is recognized quasi xe2x80x9cimmediately,xe2x80x9d the sluggish component temperatures can rise only slightly (the dome temperature remains below the permissible value). The sensor temperature drops again rapidly due to the heating air flow after the elimination of the damming. The heater is thus ready for use.
Other advantages of the sensor arrangement according to the present invention are as follows:
Overheating can also be recognized in the case of intake-side damming without discharge-side scoop.
The sensor is in the vicinity of the control device. The connection is thus very simple.
An inexpensive sensor for temperatures below 200xc2x0 C. can be used. The cable insulation must meet only slight temperature requirements.
If the evaluation of the slight temperature difference between the heating air outlet and the heating air temperature at the proposed location at different output stages is eliminated, an inexpensive thermal switch can be used. The wiring for supply and signal evaluation in the control device can thus be eliminated.
Since the sensor is arranged in the gas flow, it does not have to be assembled as an expensive component sensor. It must only be clipped, e.g., as a plate in a bracket cast integrally on the jacket shell.
Another advantageous variant consists of integrating the flame and overheating sensors or switches in one housing, and as a result more costs are saved.
The heating air temperature at the proposed measurement point is far less dependent on the output than the component temperatures; an overheating sensor, which is designed as a component sensor, must be adjusted to the maximum allowable component temperature. As a result, at low burner outputs the threshold is very far from the actual component temperatures during normal operation. Thus, this already leads to unacceptably high heating air and jacket shell temperatures in the case of complete and partial damming before the threshold is reached. This effect is avoided with the sensor according to the present invention.
The signal can be used to evaluate the degree of damming. In the case of sample inserts, it is thus possible to recognize limit cases in which critical temperatures are known to be able to be reached due to resistances of the heating air flow during operation in limit cases (at high altitudes).
The signal offers a very good possibility of utilizing the flame failure controller as a redundancy in addition to the overheating sensor, especially compared with a combination of a short heat exchanger and two component sensors. The signal difference is very small in these combinations due to the short distance, so that the overheating evaluation may fail in the case of temperature gradients due to different time constants (e.g., due to manufacturing-related dispersions in assembly). If the sensor according to the present invention is used in conjunction with a component sensor, this effect can be reliably avoided due to the great temperature difference.